Mediators, redox catalysis

A general theory based on the quantitative treatment of the reaction layer profile exists for pure redox catalysis where the crucial function of the redox mediator is solely electron transfer and where the catalytic activity largely depends only on the redox potential and not on the structure of the catalyst This theory is consistent... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

Chapters 4 and 5 are devoted to molecular and biomolecular catalysis of electrochemical reactions. As discussed earlier, molecular electrochemistry deals with transforming molecules by electrochemical means. With molecular catalysis of electrochemical reactions, we address the converse aspect of molecular electrochemistry how to use molecules to produce better electrochemistry. It is first important to distinguish redox catalysis from chemical catalysis. In the first case, the catalytic effect stems from the three-dimensional dispersion of the mediator (catalyst), which merely shuttles the electrons between the electrode and the reactant. In chemical catalysis, there is a more intimate interaction between the active form of the catalyst and the reactant. The differences between the two types of catalysis are illustrated by examples of homogeneous systems in which not only the rapidity of the catalytic process, but also the selectivity problems, are discussed. 

Immobilizing the catalyst on the electrode surface is useful for both synthetic and sensors applications. Monomolecular coatings do not allow redox catalysis, but multilayered coatings do. The catalytic responses are then functions of three main factors in addition to transport of the reactant from the bulk of the solution to the film surface transport of electrons through the film, transport of the reactant in the reverse direction, and catalytic reaction. The interplay of these factors is described with the help of characteristic currents and kinetic zone diagrams. In several systems the mediator plays the role of an electron shuttle and of a catalyst. More interesting are the systems in which the two roles are assigned to two different molecules chosen to fulfill these two different functions, as illustrated by a typical experimental example. 

Redox catalysis, which will not be discussed in detail herein, consists of lowering the kinetic barrier of the reduction (or the oxidation) process of a species, which thermodynamically is little inclined to be reduced (or oxidized), by use of a redox mediator. The latter has the role of carrying electrons towards (or away from) the low redox-active original species. 

Preparation of photo-active and redox-active dendritic macromolecules, which undergo simultaneous muitielectron transfer is also attractive. Considering the potential applications of these multimetallic dendrimers as electron transfer mediators in redox catalysis, photoinduced electron transfer, and molecular electronics, new interesting results can be awaited in the near future. 

Scheme 9. Products of perturbed redox catalysis witli pyrene as mediator...

The indirect electrochemical cleavage of halides has been studied in detail by Lund and Simonet et al. -272) course of the reaction is strongly influenced by the structure of the substrates. Thus, aryl and benzyl halides do not form alkylation products of the mediators. The products Of the perturbed redox catalysis are, however, favored in the case of aliphatic halides. Primary halides give predominantly monoalkylation products, while tertiary halides favor the formation of dialkylation products. 

In studies of analogs of the redox cofactor pyrroloquinoline quinone (PQQ), synthetic efforts have focused initially on isosteric, isomeric structures that reflect on important mechanisms of electron-transfer catalysis mediated by PQQ. These studies provide insight into the choice of PQQ as an electron-transfer catalyst in nature, and bear directly on pharmaceutical applications of this vitamin-like nutritional factor. 

The second alternative to bypass a difficult RX reduction consists of using redox catalysis [29], Thus, the reduction of RX can be performed at the much less negative one-electron reversible reduction potential (Equation 12.18) of an adequate redox mediator M, which delivers the electron to RX through an homogeneous electron transfer when RX does exist (Equation 12.19), or for a concerted bondbreaking RX reduction (Equation 12.20) ... 

In all of the examples considered, Ei/2 of the acceptor was much more negative than that of the donor. However, in liquid phase one-electron transfer from a donor to an acceptor can proceed even with an unfavorable difference in the potentials if the system contains a third component, the so-called mediator. The mediator is a substance capable of accepting an electron from a donor and sending it instantly to an acceptor. Julliard and Chanon (1983), Chanon, Rajzmann, and Chanon (1990), and Saveant (1980, 1993) developed redox catalysis largely for use in electrochemistry. As an example, the reaction of ter-achloromethane with /V,/V,/V ,Af-tetramethyl-p-phenylenediamine (TMPDA) can be discussed. The presence of p-benzoquinone (Q) in the system provokes electron transfer (Sosonkin et al. 1983). Because benzoquinone itself and tetrametyl-p-phenylenediamine interact faintly, the effect is evidently a result of redox catalysis. The following schemes reflect this kind of catalysis ... 

In the above two independent studies, the feasibility of CPMV as a nanobuilding block for chemical conjugation with redox-active compounds was demonstrated. The resulting robust, and monodisperse particles could serve as a multielectron reservoir that might lead to the development of nanoscale electron transfer mediators in redox catalysis, molecular recognition, and amperometric biosensors and to nanoelectronic devices such as molecular batteries or capacitors. 

Initiation by electrochemical induction may have the disadvantage of low yields of substitution due to the reduction of the radicals formed near the electrode, mainly in those cases in which the radical anion of the halide compound fragments at a considerably high rate. Redox catalysis, that is activation involving a suitable ET mediator, is an important means to avoid termination steps in electrochemically induced reactions. This approach has been extensively studied by the Saveant group15. A general equation has been proposed in order to predict the yield of ET-initiated S l chain reactions and related mechanisms under preparative electrochemical conditions in the presence of a redox mediator35. 

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

Redox Catalysis or Mediated Electron Transfer EC Mechanism. 

Compare the half-factor in Eq. (205) or the half-exponent in Eq. (206.] This effect, which arises from the heterogeneous nature of the electrochemical process (i.e., a surface reaction vis-a-vis a volume reaction in homogeneous phases ), is the basis of the efficiency of redox catalysis or mediated electron transfer (see Sec. III.E.3 and also Chapter 28 mainly devoted to this topic). Thus for a given redox system, as in the sequence in Eqs. (190) and (191), the use of a redox mediator M in Eq. (207) allows the reduction of R to be performed at potentials less cathodic than x/i in Eq. (205) (or the R oxidation at potentials less anodic than E1/2) for the same electrochemical setup (i.e., an identical mass transfer rate). 

Another application of redox catalysis is the study of dissociative electron transfer reactions (Scheme 1). The resulting free radical R may undergo either of two reactions, coupling with the mediator radical anion (iii) or reduction to R (iv) [128] (Scheme 8). The coupling reaction is usually considered as unwanted since the mediator is irreversibly consumed in this step. The reaction is, however, synthetically useful [128],... 

At a potential of 1.1 to 1.3 V (vs. Ag/Ag+ 0.1 M), (A-benzylaziridine is opened to form a cyclic tetramer, which may be explained by an electron transfer chain mechanism [31]. The same reaction may also be performed by redox catalysis using tris(4-bromophe-nyOamine as mediator at a potential of about 0.75 V (vs. Ag/Ag+). The possibility of catalysis by electrogenerated acid was excluded. 

The term redox catalysis is used when the redox couple P/Q (implying an electron carrier as mediator) merely plays the role of an electron carrier to substrate (X) mainly in homogeneous phase (without any formation of an adduct between the organic species and the catalyst). The electron exchange between the two soluble entities Q and X occurs only by means of a so-called outer sphere process (see Chapter 2). It is for example, the case for indirect reduction of aromatic halides by radical anions electrogenerated from a properly chosen mediator (for instance, an aromatic hydrocarbon or a ketone). 

In the following, examples of indirect electrochemical reactions (mainly redox catalysis) with an organic mediator are given. 

Table 1 Examples of Mediators Currently Used in Redox Catalysis Experiments With Their Redox Potentials Established in DMF Containing 0.1 M TBABF4, with Redox Potentials Referred to the Aqueous SCE...

Table 2 Examples of Indirect Conversions by Means of Organic Mediators — Reductive Redox Catalysis... 

Indirect reductions are also permitted because species (radical anions and/or dianions) formed after homogeneous electron transfer are so basic that they are rapidly protonated by solvent or any acidic impurities. Thus, dienes and trienes like allocimene and dimethyl 2,3-butadiene may afford a redox catalysis processes [42] in dimethylforma-mide (DMF) in the presence of naphthalene as a mediator. 

A more complicated variation of the EC scheme, largely studied by voltammetry, is the situation where reaction (12.3.28) is reversible, but the product Y is unstable and undergoes a fast following reaction (Y X). This instability of Y tends to drive reaction (12.3.28) to the right, so the observed behavior resembles that of the Ei-Cj scheme. In this case, the 0/R couple mediates the reduction of species Z, with the ultimate production of species X, and the process is called redox catalysis. By selecting a mediator couple whose lies positive of that of the Z/Y couple and noting changes in the cyclic voltammetric response with v and the concentration of Z, one can find the rate constant for the decomposition of Y to X, even if it is too rapid to measure by direct electrochemistry of Z (i.e., as an EC reaction) (8, 9). 

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